1. Field of the Invention
The present invention relates to a method of operating an injection molding machine including an injection cylinder for plasticizing injection material and a die to be filled with the injection material which has been plasticized by the injection cylinder, in which the injection material which has been preheated is supplied to the injection cylinder by a predetermined amount thereby to plasticize the material, and also relates to the injection molding machine.
2. Description of the Related Art
An injection molding machine includes an injection device and a die device. The injection device includes an injection cylinder, a screw which is provided in the injection cylinder so as to be driven in a rotary direction and in an axial direction, and a hopper for supplying injection material into the injection cylinder. When the injection material is supplied from the hopper to the injection cylinder by a predetermined amount while the screw is driven to rotate, the injection material is subjected to both frictional force and shearing action, and generates heat. Moreover, a heater is provided on an outer periphery of the injection cylinder, and therefore, the injection material is molten with the heat transmitted from the injection cylinder, and the heat generated by the friction, shearing action and so on, and then, accumulated in a measuring room in a distal end part of the injection cylinder.
On the other hand, the die device includes a fixed side die which is attached to a fixed platen, a movable die which is attached to a movable platen and is adapted to be clamped or opened with respect to the fixed platen, and a die clamping device for clamping the movable platen with respect to the fixed platen. Moreover, a cavity for imparting a shape to a molded product is formed on a die mating face, that is, a parting face between the movable side die and the fixed side die. Further, the die device is provided with a die temperature regulating machine.
Therefore, the injection material in a molten state having high temperature which has been measured, as described above, is injected to be filled in the cavity of the die which has been clamped by driving the screw in the axial direction. Then, by opening the movable side die, after the injection material has been cooled and solidified, the molded product is obtained. On this occasion, for the purpose of shortening a molding cycle, the die filled with the injection material having the high temperature is cooled with thermal medium which is supplied to and discharged from the die temperature regulating machine.
JP-A-2002-273771 discloses such an injection molding die that temperature of the die can be regulated. Specifically, there is shown the die which can be controlled by die temperature regulating means, so that temperature of a cavity of the die before it is filled with the injection material may become higher near an inlet of the molten resin and become lower at a deep side. In this document, there is no description concerning utilization of heat of thermal medium which has become high temperature after it has cooled the molten resin.
JP-A-2008-246940 discloses an injection molding machine which is provided with a clean booth chamber so as to surround a die part, for the purpose of preventing contamination of a molded product. An air supply unit for blowing out clean air is provided in the clean booth chamber. Moreover, the clean booth chamber is open to the exterior through a delivery port for delivering the molded product from the clean booth chamber to the exterior, and a through hole which a fixed platen passes through. The air having high temperature in the clean booth chamber flows out from the through hole. Then, the flowing air having the high temperature is circulated to the air supply unit.
JP-A-H05-253993 discloses a heat recovery device provided with a space for supplying and discharging regenerative material, which is formed in at least one of a fixed side die and a movable side die. The space is connected to a regenerative material supply tank and a regenerative material storage tank which are provided outside the die, by means of respective pipes. Therefore, when the regenerative material is supplied into the space from the regenerative material supply tank, heat is exchanged by way of the dies between the regenerative material and molten resin having high temperature which has been filled. As the results, the temperature of the regenerative material is raised. By storing the regenerative material which has become high temperature, in the regenerative material storage tank, it is possible to utilize the regenerative material for raising the temperature of the dies prior to the injection.
JP-A-H09-262885 discloses an injection molding machine including an injection cylinder in which a screw is provided so as to be driven in a rotary direction and in an axial direction, and a hopper for supplying resin material to the injection cylinder. In the injection molding machine, a plurality of heaters are provided on an outer periphery of the injection cylinder, and a cover having cooling fans at a predetermined interval is provided on outer peripheries of the heaters. An interior of the cover having the cooling fans is formed as a heat recovery chamber. The heat recovery chamber is connected to the hopper by means of a hot air passage.
In the injection molding machine disclosed in JP-A-2008-246940, it seems that the heat exhausted from the die is recovered. However, the hot air leaked from the clean booth chamber is simply returned to the clean booth chamber, and it cannot be said, in the strict sense, that the exhausted heat is recovered. According to the heat recovery device disclosed in JP-A-H05-253993, because the heat is exchanged between the regenerative material in the space and the molten resin having the high temperature by way of the dies, it is recognized that the heat of the molten resin is recovered to the regenerative material, and the exhausted heat is effectively utilized. However, there are various problems in putting the heat recovery device into practice. For example, there is such a problem that cost for the die is increased, because the space must be especially provided inside the die. Moreover, the device cannot be applied to an existing die as it is, because the space must be especially provided inside the die. Further, a position of the space may be restricted depending on a layout of the cavity. Still further, heat exchanging efficiency of the space is low due to its shape. In case where a layout of the space is restricted, the efficiency is further lowered.
According to the injection molding machine disclosed in JP-A-H09-262885, the heat recovery chamber is formed inside the cover having the cooling fans. Because the heat recovery chamber is connected to the hopper by means of the hot air passage, when the outside air is introduced into the heat recovery chamber, by actuating the cooling fans according to necessity, the injection cylinder can be cooled, and the resin material which is contained in the hopper can be preheated and dried with the hot air obtained from the air which has cooled the injection cylinder. As the results, plasticizing performance can be enhanced, and plasticizing operation can be achieved, even though the molding cycle is shortened. However, there still remain those matters to be improved. The reason will be described herein below, referring to a motor-driven injection molding machine, as an example. Table 1 shows distribution of energy which is supplied per one molded product in a certain molding cycle of the motor-driven injection molding machine. As shown in the table 1, about 70% of electric energy is used for plasticizing the resin in a solid state.
TABLE 1An example of a certain molding cycleMolding cycle time = 30 sec. Material: PPJcylCylinderJhlosJmchJkasheatJmlosJalosJinjHopperJcontMechanicalProcessPlasticizingradiationMotorAmplifierInjectionwaterControllossEnergy (J)2694214508654625462241781050050409069Distribution72.012.01.51.56.52.81.32.4of energy (%)Total Energy: 374218 (J)
The molten resin which has been plasticized is injected and filled into the die. Then, after a cooling and solidifying process, the molded product is taken out. The resin in the molten state which has absorbed a large amount of electric power, that is, heat radiates a large amount of heat during the cooling and solidifying process. However, this large amount of heat is not recovered in the injection molding machine disclosed in JP-A-H09-262885. Calculating from data concerning polypropylene in the database of thermal characteristics of plastics, an amount of heat required for heating and melting 1 g of polypropylene at 40° C. to 215° C. is 592.1 J/g, and an amount of heat required for heating and melting 1 g of polypropylene at 50° C. to 215° C. is 539.0 J/g. When it has been preheated to 50° C., energy saving of about 10% is attained. Because a room temperature at a time of general molding is 25° C., energy saving of about 30% is estimated, when it has been preheated by 30° C. However, in the injection molding machine disclosed in JP-A-H09-262885, the heat is recovered from the injection cylinder. In other words, the heat is not recovered from a part where the molten resin is forcibly cooled, and hence, the resin cannot be preheated to such a high temperature as described above. Specifically, in the invention disclosed in JP-A-H09-262885, only the heat which leaks to the exterior from the heaters wound around the outer periphery of the injection cylinder is recovered, but a large amount of heat which is radiated when the molten resin is solidified is not utilized.